1. Field of the Invention
The invention relates to a deformable inexpensive crucible for use with a dynamic thermo-mechanical test or simulating system, specifically such a crucible for holding a self-resistively heated specimen at liquid temperatures while the specimen is being controllably deformed in compression or tension, and one preferably, though not exclusively, suited for use in simulating thin-strip continuous casting processes.
2. Description of the Prior Art
Generally speaking, steel materials are fabricated in a mill through a multi-step process. Once the necessary raw materials, including pig iron, are gathered, molten steel is first cast into a desired starting shape. During a casting process, pig iron is typically heated to liquefaction, and appropriate alloying and other raw materials are then added to produce a desired composition. The resulting liquid metal, i.e. liquid steel, is then poured into a mold and cast into a desired shape. With current equipment, casting is done on a continuous basis, such that a single strand of metal is produced from liquefied steel. This strand may take the form of a slab, sheet, wire or other special shape, such as I-beams, angles or channels, as defined by a mold then in use. During continuous casting, the walls of the mold which confine the liquefied steel are cooled to a temperature below the solidification temperature of the steel. As a result, as the external surfaces of the strand, which abut against the walls of the mold, freeze while the central region of the strand remains liquid. Through solidification, the surface provides sufficient strength to the strand to maintain its shape. Inasmuch as liquid metal can be continuously poured into the mold from a tundish (an intermediary vessel from which liquid steel is directed into the mold) while the strand exits from the mold, the strand takes on a continuous shape. Once cast and appropriately cooled, the strand may be coiled, if in the form of sheet or wire, or cut to a desired length(es), if the strand is in the form of a thick slab or other special shape.
After a strand is fabricated, it is then typically rolled to a desired intermediate thickness, generally through a stand of several rolling mills, typically using both hot and cold rolling processes. Each mill imparts a given reduction in thickness to the strand while increasing its length. This overall rolling process may provide a considerable reduction in thickness. For example, for strands on the order of 10 inches (approximately 25 cm) in thickness, a roll stand may well reduce this strand to sheet only 0.04 inch (approximately 0.1 cm) thick, for a total reduction of 250. For ease of reference, this process will be referred to as "thick-slab" casting. Unfortunately, rolling mills, particularly those intended for rolling relatively thick materials, are physically huge and inordinately expensive, with each particular roll stand consuming a substantial amount of energy. Moreover, since various rolling characteristics change the underlying morphology of the rolled material and thus affect the physical properties of the resulting metallic sheet, such as in terms of material temperature, sheet strain and strain rate and sheet flatness, each roll stand must be precisely controlled.
Consequently, given the difficulties in reducing relatively thick strands of continuously cast steel, stainless steel or other such materials, the art has turned to the use of so-called thin-strip continuous casting. This form of casting produces strands that are considerably thinner than those previously used, typically on the order of only 10% or less than that of a thick slab, such as a strip 1 inch thick (approximately 2.5 cm) or less rather than 10 inches as had occurred previously with thick slab casting. The exiting cast strip from a continuous caster is typically "semi-solid", i.e. a portion of the cast material, typically its outer surfaces, is solid while the remainder is liquid. Deforming a semi-solid material can be advantageously accomplished quite easily and with substantially less energy than deforming the same sized, though solid, strip.
Thus, by using relatively thin strands, such as, e.g., a one inch (approximately 2.5 cm) thick cast metallic strip, significant savings accrue to the mill owner in terms of needing fewer roll stands and associated production equipment, less physical space, less deformation and hence substantially less energy to roll the strand to the desired thickness than with thick-slab casting. Furthermore, as contrasted with thick-slab cast materials, thin-strip casters, by virtue of producing thin strip at relatively high lineal rates and hence, subjected to relatively high cooling rates (to properly freeze the surfaces), advantageously produces final material with a relatively small size grain and without a need for subsequent heat treatment. Consequently, steel manufacturers prefer using thin-strip rather than thick-slab continuously cast materials.
However, not only does thin-strip casting exacerbate problems already inherent in thick-slab casting, it also presents a variety of new problems. The former includes changes in material segregation brought about by inclusions of foreign materials in the cast strand as well as by high rates of solidification.
Since thin-strip casting relies on using fewer rolling stands to reach a final sheet thickness than does thick-slab casting processes, much less mechanical work is imparted to a thin strand than a thick slab. Inasmuch as each reduction changes and further refines the microstructure of the strand, relatively few roll stands, as used in a thin-strip casting process, yields decreased grain refinement over that occurring in a thick-slab casting process. Consequently, owing to a lessened number of roll stands, the desired properties of the final strip must be developed by changing some of variables in the casting process itself along with those applicable to the roll stand that imparts a final reduction.
In order to properly set the appropriate process variables, the dynamics of the thin-strip continuous casting and rolling processes must be properly understood. Doing so entails gaining knowledge of the thermal and mechanical properties of the cast material while it is solidifying, including the time the material is semi-solid, and thus how, during that time, the material can be treated, thermally and/or through deformation, to yield a desired set of properties. For example, the relationships of time at temperature, as well as temperature itself and solidification rate, to resulting material properties are all critically important.
In practice, "thin-strip" continuous casting (as with other physical manufacturing processes, such as rolling or forging, utilize very costly production equipment, which, for economic considerations, often requires that this equipment be commercially and profitably used as much as possible--with little, if any, use made available for research and experimentation. As a result, sufficient operational data on, inter alia, thin-slab continuous casting processes, while essential for proper understanding and eventual control of the process, has proven to be very costly to obtain from actual production equipment.
In the past, the other physical manufacturing processes presented the same commercially-induced difficulties in readily providing cost-effective process data. Accordingly and with respect to those processes, the art has turned to employing equipment which accurately and dynamically, and highly cost-effectively, simulates the operation of such a process but on a relatively small metallic specimen, measures the consequent physical affect(s), e.g. dilation, on the specimen that results from the simulated process and then provides resulting specimen measurement data for subsequent inspection and analysis. In this instance, the specimen takes the place of a considerably larger workpiece; the simulation equipment functions as a considerably scaled down, though accurately controllable, version of the desired process. Provided the simulation is sufficiently accurate, the data will reflect the affect(s) of the corresponding physical manufacturing process(es) on a metallic workpiece. One such piece of equipment, which provides very accurate, dynamic thermo-mechanical simulation and testing of a specimen, is the GLEEBLE 2000 system (manufactured by the present assignee which also owns the registered trademark "GLEEBLE"). This system, under computer control, self-resistively heats a metallic specimen according to a desired thermal program to establish, in the specimen, a desired longitudinal thermal profile with transverse iso-thermal planes. Simultaneously, the system imparts a desired deformation profile to the specimen and obtains physical measurement data, such as, e.g., dilation, therefrom. In simulating a physical manufacturing process, the simulator is programmed to simultaneously impart the same thermal and mechanical deformation profiles to the specimen as the process, e.g., rolling, forging or extruding, would be expected to impart to a workpiece. Hence, the specimen would exhibit, though at a reduced scale, the same temperature gradients and strain direction(s) as the process would be expected to produce in the workpiece. Advantageously, not only is simulation highly cost-effective, generally by several orders of magnitude, as compared to use of actual production equipment in a test environment and the associated cost of "down time", but also a simulator allows many more variables to be selectively and accurately controlled and then precisely measured than would be possible on actual production equipment. This latter factor often permits improvements to be made in the process, resulting material or both.
In the GLEEBLE 2000 system, a solid specimen is longitudinally held between two opposing electrically conductive jaws or anvils. One of the jaws is controllably and longitudinally moveable with respect to the other to impart a desired axial tensile/compressive force, resulting in a desired deformation profile, to the specimen. Coincident with such movement or not, a controlled electrical heating current can be serially passed from one jaw, through the specimen to the other jaw to self-resistively heat the specimen to a desired thermal profile.
With the relatively recent and pronounced interest in using a thin-strip continuous casting process and the pronounced advantages, including economic, of using simulation, the steel industry, inter alia, is attempting to employ dynamic thermal-mechanical physical simulators to characterize this process in much the same manner as these simulators have been used to assess other physical processes, such as rolling.
However, with specimens existing in a semi-solid state as would occur during simulated thin-strip continuous casting, a serious problem arises as to how to hold the specimen within the physical simulator, such as the GLEEBLE 2000 system. Part of the time the specimen is in the semi-solid state, a workzone of the specimen may be partly liquid and partly solid, or entirely liquid. While the workzone remains in either of these states, the specimen must be supported in some fashion. Furthermore, since semi-solid strip exiting from a continuous thin-strip caster is often rolled to a given thickness, the specimen must be held, while still semi-solid, such that it can be deformed. Doing so would typically entail holding the specimen in some manner such that as the specimen were to be compressed (squeezed) along one direction, the specimen could expand along another direction perpendicular to the direction of the force used for compression. Inasmuch as the specimen is semi-solid, the compressive force would be very small; as contrasted with a relatively large force required to compress the specimen while it was totally solid.
This problem is compounded by the need for such a crucible that not only holds the specimen but simultaneously permits it to be self-resistively heated such that, e.g., the GLEEBLE 2000 system can be used to simulate the thin-strip continuous casting process.
Clearly then, what is needed for simulating thin-strip continuous casting processes is a proper crucible: one that appropriately supports a semi-solid self-resistively heated specimen and surrounds enough of it to hold the specimen shape before, during and after the simulation. Conventional high-temperature crucibles known in the art for use with liquid metallic specimens were manufactured from quartz or other materials, such as suitable ceramics, that remained solid at temperatures well above the melting point of the specimen.
Unfortunately, conventional high-temperature crucibles suffer various serious drawbacks which either completely or at least partially preclude their use with thin-strip continuous casting simulations.
First, crucibles made from, e.g., quartz and ceramic based materials, are brittle and readily fracture when deformed. Accordingly, if such a crucible were to hold a molten or partly melted specimen, then, when subsequently deformed--even slightly, the crucible would fracture and a liquid portion of the specimen would spill from the fractured crucible. Hence, once the crucible fractured, the simulation would be abruptly halted and any simulation results, based on further deformation, frustrated. Moreover, quartz crucibles also tend to be quite expensive, typically on the order of approximately $10.00 or more each.
Second, since the simulation involves deforming the semi-solid specimen, each individual simulation would also destroy the crucible. While various high temperature metals, such as tantalum, exist in the art and from which a crucible could be formed, particularly one that will deform well and not fracture, these metals are also very expensive. For example, one crucible constructed from sufficiently thin tantalum sheet may cost upwards of $50.00 or more. Properly assessing the behavior of a casting process may involve use of a significant number of specimens, often hundreds if not more. Clearly, if expensive crucibles were to be used, with one per each specimen, the added cost of the crucibles themselves may inhibit extensive simulation, over large numbers of specimens, and thus economically preclude completely characterizing the casting process of interest, ultimately to the detriment of the mill owner.
Therefore, while a pressing demand exists for equipment that can accurately and cost-effectively simulate a continuous thin-strip casting process, a specific problem in the art must first be solved before that demand can be addressed: a low-cost, deformable crucible must be provided to properly and completely hold a self-resistively heated specimen, at liquid temperatures with a semi-solid or liquid portion, before, during and after the simulation. Advantageously, such a crucible would readily permit dynamic thermo-mechanical simulating and test systems, such as the GLEEBLE 2000 system, to be used in accurately simulating thin-strip continuous casting processes.